BMediaEventLooper

Derived from: BMediaNode
Declared in:  be/media/MediaEventLooper.h
Library: libmedia.so
Allocation: Constructor only
Summary:  more...

The BMediaEventLooper class provides a control thread that automatically queues received media events and calls the HandleEvent() function you implement to process them as necessary. This takes a lot of the drudgery out of writing nodes:

• You don't have to write a service thread to receive and process incoming messages; BMediaEventLooper does this for you (much like a BLooper, hence the similarity in names).

• You don't have to write code to queue up events until it's time to handle them, since BMediaEventLooper maintains a BTimedEventQueue for you. Just push events onto the queue as you receive them and the BMediaEventLooper pops them off for you at the appropriate times and asks you to handle them.

• If all your node needs to do is know when Start(), Stop(), and Seek() requests come due, your node doesn't even have to override these functions—BMediaEventLooper automatically intercepts them and pushes them onto the queue for you.

• The BTimedEventQueue employed by BMediaEventLooper automatically provides support for queueing multiple start, stop, and seek events.

BMediaEventLooper maintains two queues: one for real-time events, and one for normal events. Events in the real-time queue are handled according to their real time, while events in the other queue are handled based on their performance time. So the values of the time stamps on events in these two queues are handled differently; keep this in mind if you need to peek into the queues yourself.

Implementing a Node (the Easy Way)

Although BMediaEventLooper does take away a lot of the complicated node construction work, there's still work to be done. There are still plenty of hooks you have to implement yourself (which ones, exactly, depend on whether you're a producer or a consumer).

You can study a specific example of how to implement a node using BMediaEventLooper by reading "A BMediaEventLooper Example".

Hook Functions

• CleanUpEvent()

• ControlLoop()

• HandleEvent()

• OfflineTime()

Constructor and Destructor

BMediaEventLooper()

protected:

explicit BMediaEventLooper(uint32 apiVersion = B_BEOS_VERSION)

You need to override this function to handle your node's initialization needs. The apiVersion argument indicates the version of the BeOS API the object was written for; you should let the default value be used—this will cause your node to inherit the API version of the BeOS version you're compiling under.

~BMediaEventLooper()

protected:

virtual ~BMediaEventLooper(void)

Calls Quit() to stop the control thread and free allocated memory.

Member Functions

BufferDuration() see SetBufferDuration()

CleanUpEvent()

protected:

virtual void CleanUpEvent(const media_timed_event *event)

Implement this function to clean up after custom events you've created and added to your queue. It's called when a custom event is removed from the queue, to let you handle any special tidying-up that the event might require.

ControlLoop()

protected:

virtual void ControlLoop(void)

This function waits for messages, pops events off the queue, and calls DispatchEvent(). It's called automatically when the BMediaEventLooper is Run().

If you choose to reimplement this function, be very careful; it's very easy to cause Bad Things to happen.

ControlThread()

thread_id ControlThread(void)

Returns the control thread's thread_id.

DispatchEvent()

protected:

void DispatchEvent(const media_timed_event *event,       bigtime_t lateness, bool realTimeEvent = false)

Calls HandleEvent() to let your code handle the specified event. If your code doesn't handle it, this function may have a default handler to process it. In general, you won't call this function.

BMediaEventLooper compensates your performance time by adding the event latency (see SetEventLatency()) and the scheduling latency (or, for real-time events, only the scheduling latency).

It's the control loop's job to remove the event from the queue; this function doesn't do that.

EventLatency() see SetEventLatency()

EventQueue() , RealTimeQueue()

protected:

BTimedEventQueue *EventQueue(void) BTimedEventQueue *RealTimeQueue(void)

EventQueue() returns a pointer to the BTimedEventQueue used by the message handler. RealTimeQueue() returns a pointer to the BTimedEventQueue used to queue real-time events.

Events in the real-time queue are handled according to their real time, while events in the normal event queue are handled based on their performance time. So the values of the time stamps on events in these two queues are handled differently; keep this in mind if you need to peek into the queues yourself.

HandleEvent()

protected:

virtual void HandleEvent(const media_timed_event *event,       bigtime_t lateness, bool realTimeEvent = false)

Implement this function to handle incoming media events. The event argument references a media_timed_event structure that describes the event. lateness indicates how late the event is, and realTimeEvent is true if the event needs to be handled in real time.

The BMediaEventLooper will call this function from the DispatchEvent() function. It's the control loop's job to remove the event from the queue.

OfflineTime() see SetOfflineTime()

NodeRegistered()

protected:

virtual void NodeRegistered(void)

The Media Server calls this hook function after the node has been registered. This is derived from BMediaNode; BMediaEventLooper implements it to call Run() automatically when the node is registered; if you implement NodeRegistered() you should call through to BMediaEventLooper::NodeRegistered() after you've done your custom operations.

Priority() see SetPriority()

Quit()

void Quit(void)

Closes the node's control port and closes the control thread. Blocks until the control thread is gone.

RealTimeQueue() see EventQueue()

Run()

void Run(void)

Spawns and runs the control thread; this is called automatically by the default NodeRegistered() implementation. If you override NodeRegistered(), be sure you call through to the default implementation, or call Run().

RunState() see SetRunState()

SchedulingLatency()

protected:

bigtime_t SchedulingLatency(void) const

Returns the scheduling latnecy, in microseconds, of the node.

SetBufferDuration() , BufferDuration()

protected:

void SetBufferDuration(bigtime_t duration) bigtime_t BufferDuration(void) const

SetBufferDuration() sets the duration of the node's buffers. The duration is clamped to 0 if it's less than 0.

BufferDuration() returns the duration of the nodes' buffers.

SetEventLatency() , EventLatency()

protected:

void SetEventLatency(bigtime_t latency) bigtime_t EventLatency(void) const

SetEventLatency() sets the event latency. The event latency is the upstream and downstream algorithmic latency of your node—but should not include scheduling latency. This latency is taken into account by the BMediaEventLooper when deciding when to pop events off the queue for you to process.

EventLatency() returns the event latency.

SetOfflineTime() , OfflineTime()

protected:

void SetOfflineTime(bigtime_t offlineTime) virtual bigtime_t OfflineTime(void)

SetOfflineTime() sets the time that OfflineTime() will return.

Augment OfflineTime() to compute the node's current time; it's called by the Media Kit when it's in offline mode. Update any appropriate internal information as well, then call through to the BMediaEventLooper implementation.

SetPriority() , Priority()

protected:

void SetPriority(int32 priority) int32 Priority(void) const

SetPriority() sets the control thread's priority. Values less than 0 are clamped to 0, and values greater than 120 are clamped to 120.

Priority() returns the control thread's current priority.

SetRunState() , RunState()

protected:

void SetRunState(run_state state) run_state RunState(void) const

SetRunState() sets the node's current run state.

RunState() returns the current run state.

See also: run_state

Constants

run_state

Declared in:  be/media/MediaEventLooper.h

Constant	Description
B_IN_DISTRESS	The node is experiencing problems.
B_UNREGISTERED	The node hasn't been registered with the Media roster yet.
B_STOPPED	The node isn't running.
B_STARTED	The node is running.
B_QUITTING	The node's in the process of shutting down.
B_TERMINATED	The node has been terminated.
B_USER_RUN_STATES	Base for user-defined run states.